U.S. Pat. No. 4,806,774 of Lin et al. describes an optical inspection system for detecting nonperiodic pattern defects in a patterned specimen, such as a wafer of the type employed in the large-scale manufacture of semiconductor devices and integrated circuits. The specimen includes many redundant circuit patterns. The inspection system employs a Fourier transform lens positioned along a system optic axis to produce a light pattern corresponding to the Fourier transform spatial frequency components of an illuminated area of the specimen wafer. A spatial filter positioned along the system optic axis intercepts the light pattern and cooperates with an inverse Fourier transform lens to produce an image pattern of nonperiodic defects located in the illuminated area of the wafer.
The spatial filter comprises a two-dimensional Fourier transform pattern of opaque and light-transmitting regions representing a substantially error-free wafer against which the specimen wafer is compared. The spatial filter may be fabricated by exposing a recording medium, such as a photographic plate, to light diffracted primarily by the illuminated redundant circuit patterns on the specimen wafer. The spatial frequency components corresponding to an error-free wafer are blocked by the opaque regions of the spatial filter, and the spatial frequency components corresponding to the defects in the specimen wafer are largely transmitted through the light-transmitting regions of the spatial filter to be processed for error detection.
Spatial filters employed in such a system suffer from at least three disadvantages. First, each filter employed in these systems corresponds only to a specific photomask configuration. As a result, a new filter must be manufactured for each wafer pattern to be inspected. Second, photographic or theory-based processes for manufacturing the spatial filters are relatively slow and limit the rate at which inspections may be performed. Third, replenishment of the materials for forming a spatial filter for each configuration increases the operating costs of the inspection system.